Memory devices are typically provided as internal, semiconductor, integrated circuit devices in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory.
Flash memory has developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Changes in threshold voltage (Vt) of the memory cells, through programming (which is often referred to as writing) of charge storage structures (e.g., floating gates or charge traps) or other physical phenomena (e.g., phase change or polarization), determine the data state (e.g., data value) of each memory cell. Common uses for flash memory and other non-volatile memory include personal computers, personal digital assistants (PDAs), digital cameras, digital media players, digital recorders, games, appliances, vehicles, wireless devices, mobile telephones, and removable memory modules, and the uses for non-volatile memory continue to expand.
In programming memory, memory cells may generally be programmed as what are often termed single-level cells (SLC) or multiple-level cells (MLC). SLC may use a single memory cell to represent one digit (e.g., bit) of data. For example, in SLC, a Vt of 2.5V might indicate a programmed memory cell (e.g., representing a logical 0) while a Vt of −0.5V might indicate an erased cell (e.g., representing a logical 1). An MLC uses more than two Vt ranges, where each Vt range indicates a different data state. Multiple-level cells can take advantage of the analog nature of a traditional charge storage cell by assigning a bit pattern to a specific Vt range. While MLC typically uses a memory cell to represent one data state of a binary number of data states (e.g., 4, 8, 16, . . . ), a memory cell operated as MLC may be used to represent a non-binary number of data states. For example, where the MLC uses three Vt ranges, two memory cells might be used to collectively represent one of eight data states.
In programming MLC memory, data values are typically programmed one digit at a time. For example, in four-level MLC, a first digit, e.g., a least significant bit (LSB), often referred to as lower page (LP) data, may be programmed into the memory cells, thus resulting in two (e.g., first and second) threshold voltage ranges. Subsequently, a second digit, e.g., a most significant bit (MSB), often referred to as upper page (UP) data may be programmed into the memory cells, typically moving some portion of those memory cells in the first threshold voltage range into a third threshold voltage range, and moving some portion of those memory cells in the second threshold voltage range into a fourth threshold voltage range. Similarly, eight-level MLC may represent a bit pattern of three bits, including a first digit, e.g., a least significant bit (LSB) or LP data, a second digit, e.g., middle page (MP) data, and a third digit, e.g., MSB or UP data. In operating eight-level MLC, the LP data may be first programmed to the memory cells, followed by the MP data, then followed by the UP data.
In general, there is a continuing desire to provide increasing levels of memory density. One approach to increasing memory density is to program memory cells as MLC, and to increase the number of levels to which the memory cells are programmed, thus increasing the number of data states that can be represented by a single memory cell. This typically results in the margins between Vt ranges of adjacent data states becoming more narrow, which can lead to difficulties in accurately determining (e.g., reading) the data state of a memory cell.